Preparation of metal borides and silicides



PREPARATION OF METAL BORIDES AND lLICllDES David R. Stern, Fuilerton, and Quentin Hyde McKenna,

Whittier, Califl, assignors to American Potash & Chemical Corporation, a corporation of Delaware Application December 18, 1958, Serial No. 781,254

6 Claims. (Cl. 204-61) This is a continuation-impart of our application Serial No. 663,828, filed June 5, 1957, and now abandoned.

This invention relates to a new method for the preparation of metal borides and metal-boron alloys and metal silicides and metal-silicon alloys where the metals are those of the fourth, fifth, and sixth groups of the periodic table (i .e., -Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th, and U). In addition, the invention relates to the production of boron-silicon alloys.

The, borides and silicides of these metals are characterized by their metallic nature, high melting points, and very high hardness values. Some borides and silicides of the fourth and sixth groups exhibit good corrosion resistance up to comparatively high temperatures. As such, these compounds have potential usage in high temperature applications where metals and metal alloys are unsatisfactory.

Borides and silicides have been prepared by sintering the metals or metal hydrides with boron or silicon at high temperatures, e.g., of the order of 1000 C. Borides have been prepared electrolytically by the electrolysis of mixtures of B and the desired metal oxide in melts containing oxides of calcium, magnesium, or lithium, and a fluoride or calcium, magnesium, or lithium. Again, high temperatures were used (about 1000 C.) and difliculty was experienced in separating the boride crystals from other materials that codeposited.

Metal silicides have been prepared by the electrolysis of fused baths containing alkali fluosilicates and the respective metal oxides or metal fluorides. In this case, it is presumed that higher cell potentials were used to deposit elemental silicon and a given metal which in turn interacted to form the silicide.

Our invention involves the preparation of the borides and boron alloys and silicicles and silicon alloys of the metals such as titanium, tantalum, chromium, thorium, and the like, from mixtures of their carbides with boron carbide or silicon carbide in a fused salt electrolytic system. If a mixture of one of the aforementioned carbides and boron carbide (B 6), or silicon carbide (SiC) is made an anode and submerged in a fused salt bath containing an alkali chloride or mixture of alkali chlorides, a complex double fluoride salt of the metal and/or a complex double fluoride of boron or silicon, both the metal" and boron or silicon contained in the anodically charged carbide mixture passes into solution, leaving a skeletal carbon residue. The dissolved metal and boron or silicon are transported by the direct current to the cathode and are codeposited thereon. The metal and boron or silicon carbides behave as a consumable anode material and, upon reasonable depletion of the metal and boron or silicon contained therein, can be removed and fresh carbide source material added to maintain production of metal-boron alloys or metal silicon alloys or crystalline borides or silicides at the cathode.

The presence of at least one of the complex double fluoride compounds of the metal, boron, or silicon is found essential and is involved in the anodic processes.

Sttes Patent 2 However, the complex fluorides of the metal and the complex fluoride of boron or silicon can be used singly or simultaneously in a given system. For example, in the preparation of titanium borides the bath may contain either one or both of the compounds potassium 'fiuotitauate (K TiF and potassium fiuoborate (KBR as the essential melt components. It has been found that the complex fluoride ions (i.e., Til or BF are involved inthe anodic processes and that these anions enter into an oxidation-reduction reaction with the metal and boron carbide producing new ionic species which are subsequently reduced at the cathode to the metal and boron or silicon, as the case may be. The partly reduced complex fluoride anions are regenerated by electrolytic reoxidation at the anode thus made available for the reaction with both the metal carbide and the boron carbide orsilicon carbide. Thus, the complex double fiuoride compounds of the metals and boron act as regenerative and recyclic carriers of metal and boron or silicon contained in the anodically charged carbides and are essentially unchanged by electrolysis.

In any case, however, we do not wish to be limited to any theory of operation as pertains to anodic or cathodic reaction mechanisms in the practice of this invention. We wish to point out, however, that neither chlorine or fluorine are? formed at the anode since the electrolysis potential is kept well below that required to discharge the halogen gases C1 and F This is a fundarnetal difference between the process disclosed here and prior art electrolytic processes such as are disclosed in British Patent 753,031 of July 18, 1956.

The carbide materials can be in the form of a sintered massive elecetrode and attached directly to the positive terminal of the direct current source. A mixture of metal carbide and boron or siiicon carbide powders held in an anodically charged compartment of the electrolysis cell may also be employed when it isdesired to produce a mixed. alloy of boron or silicon and the metal providing the metal carbide; when the boronsilicon alloys are to be produced, a mixture of boron carbide and silicon carbide is employed as the anode. We prefer to use powders since the increased anode area enhances the reaction with the carrier anions and sustains high current efliciencies by limiting the thickness of the carbon residue layer, which acts as a diflusion barrier, to essentially one-half the dimensions of the carbide powder particles.

An electrolytic cell for use in practicing this invention is shown in the single figure in the drawing. The cell can be a graphite or carbon crucible 6, which is also the anode. The crucible is held in any suitable heat resistant metal or metal alloy shell 7 such. as Inconel. The shell is fitted with a lid 8 of the same material which can be bolted down on an asbestos gasket 9 to seal it off. A port through which the cathode 14 is passed and a second port 16 through which a thermocouple and an inner gas can be passed are provided in the lid as is a'gas outlet 17.

The mixture of metal carbide and boron or silicon carbide 11 is placed in the crucible and held between the crucible wall and a perforated or porous graphite barrier 12, which prevents the powders from being carried by electric field or convection currents over to the cathode. The cathode can be iron or mild steel and. is suspended vertically through the port provided in the lid into the melt inside the perforated graphite barrier. Other apparatus arrangements are possible and will be obvious to those experienced in the field of fused salt electrolysis.

In practicing our invention, the carbide source material may be a mixture containing the metal to boron or metal to silicon in the same ratio found in the known boride or silicide compounds (i.e., for TiB production, the carbon mixture preferably contains Ti to B in a ratio of 1:2).

However, a wide variety of ratios may be employed and we do not wish to be limited to these mentioned here.

Only one carrier compound need be employed, for example, either K TiF or KBF in the deposition of titanium borides. If both a metal and a boron or silicon double fluoride compound are employed, it appears that the best results in the production of the stoichiometric borides or silicides are obtained if these carrier compound mixtures also contain the metal and boron or silicon in approximately the ratio found in the stoichiometric borides or silicides. However, we do not wish to be limited to these specific ratios for either the carbide mixtures or the carrier compound mixtures.

The electrolysis is best performed at a potential which is too low (i.e., less than 3 volts) to decompose electrolytically the constituents of the various electrolyte mixtures. The temperature of operation can be in the range of about 400 to 1000" 0., depending on the composition of the fused bath and the related melting point. We have found that we prefer to operate in the range of 650 to 850 C. Current efficiencies in excess of 80% have been obtained under these generally stated conditions.

The following examples illustrate the detailed practice of the invention.

Example 1.A mixture of reagent grade KCl and NaCl consisting of 61'weight percent KCl and 39 weight percent NaCl was added to a graphite crucible cell assembly which contained a mixture of TiC and B C granules to inch in largest dimension), packed between the crucible wall and a perforated graphite cylinder. The mole ratio of Ti to B in the granular mixture of the carbides was 1.to 2.

The charged crucible was placed in an Inconel pot and the lid bolted down on an asbestos gasket. The entire assembly was heated in a crucible furnace with an argon gas sweep blanketing the crucible and its charge. When the salts were molten, a mixture of K TiF and KBF was added such that the final bath contained 20 weight percent of carrier salts. The mole ratio of Ti to B in the carrier mixture was 1 to 2.

A stainless steel rod shaped cathode was then introduced through the port provided in the cell lid and im- 'rnersed in the melt. The electrolysis was immediately initiated and the experiment conducted over the temperature range 720 to 730 C. with a DC. potential across the cell of 1.2 to 1.6 volts. Chlorine gas was not liberated during the run and there was no evolution of TiCl or BCl At the completion of the run, the cathode was raised out of the bath but cooled within the cell in the argon atmosphere.

The deposit was tightly adhered to the cathode as a spongy mass of crystals coated with lavender colored electrolyte. The salt bath cake was also lavender colored,

indicating the presence of tri-valent titanium ions.

The deposit was then broken away from the cathode and crushed. It was thoroughly washed with water and finally with cold concentrated HCl. After rinsing again with water and then acetone, the product was air dried at room temperature.

The titanium-boron product was in the form of fairly coarse crystalline agglomerates and had a grayish metallic appearance. The following representative analysis is typical of the material containing titanium and boron obtained from TiC and B C.

Upon removal of a cathode deposit a clean cathode may be introduced and the electrolysis cycle repeated a number of times until the available titanium and boron has been reduced to a level at which the current efficiency and the current carrying capacity of the system falls to an impractical level. The anode chamber residue, free carbon and a certain amount of residual TiC and B 0 can then be removed and new carbides added in the desired mole ratio of Ti to B.

It should be obvious to those experienced in electrochemical techniques that this process can be practiced on a semicontinuous basis with fresh metal and boron or silicon carbide added as the free carbon residue which floats on the melt and is mechanically removed. Cathode deposits can be periodically removed and a clean cathode inserted and the electrolysis resumed. Salt bath makeup is needed only to the extent that the electrolyte dragout contained in the deposits needs to be replaced occasionally to maintain the melt level at a workable height. This drag-out electrolyte can be recovered and recycled to the cell.

Example 2.A system in which ZrC and B C granules (V8 to /4 inchin largest dimensions) were employed as a consumable anode mixture was electrolyzed in the electrolytic cell described in the first example. The bath consisted of weight percent KCl-'NaCl eutectic (61 weight percent KCl39 weight percent NaCl) and 25 weight percent mixture of K ZrF and KBF having a Zr to B ratio of 1 to 2. The ZrC-B C mixture also coltained Zr and B in a 1 to 2 mole ratio.

The temperature of operation ranged from 750 to 850 C. at a D.C. cell potential of from 1.2 to 1.5 volts. Current efficiencies were found to exceed consistently. As in the titanium boronsystem, a crystalline product was recovered after washing in exactly the manner described in Example 1. This product also was a relatively coarse crystalline material, gray metallic in appearance. The following analytical data is typical .of the zirconium-boron products obtained by this method from ZrC and B C.

Theoretical Constituent Percent Percent for ZrBg Zirconium 80. 2 80. 82 Boron 19. 2 19. 18 Sodium 0.4

tinue production of zirconium borides at high current efficiencies. I

Both the titanium and zirconium-boron products given as Examples 1 and 2 were examined by X-ray diffraction and found to be the crystalline compounds TiB and ZrB Thus, in conjunction with the stoichiometry, the

X-ray data shows conclusively that the diborides were obtained in essentially pure form.

The following table presents the X-ray diffraction spacing data for samples prepared by the method of our invention and are compared with the ASTM values given for 'TiB and ZrB :3 3 Comparison of samples Wllh ASTM X-ray data TiBz ZIBz Sample Values ASIM V alucs Sample Values ASTM Values (1 A. I d A. I d A. I d A. I

3. 16 45 3. 2t 26 3. 50 40 3. 53 44 2. 60 71 2. 62 57 2. 73 75 2. 74 65 2. 02 106 2. 03 100 2. 16 100 6 16"} 1.62 15 1.76 13 1. 764 13 1. 508 41 1. 515 21 1. 678 23 1. 585 19 1. 359 41 1. 38 21 1. 480 23 1. 484 22 1. 439 19 1. 445 18 1. 368 12 l. 372 10 1. 275 19 .l. 278 17 1. 176 17 1. 179 16 1.083 8 1 079 15 1. 080 14 1. 034 10 1. 037 6 0. 993 16 0. 994 14 Example 3.-To illustrate the manufacture of a boronsilicon alloy, an anode consisting of an equimolar mixture of boron carbide and silicon carbide was immersed in a molten salt bath-consisting of 40% sodium chloride, 40% potassium chloride, and 10% each of potassium fluoborate and potassium fluosilicate, all by weight per cent. The bath was heated to a temperature between 680-855 C. and was electrolyzed at 1 volt. A boronsilicon alloy deposited on the stainless steel cathode. This alloy contained 36.8% silicon and 63.2% boron.

We claim:

1. A process for the preparation of an alloy of an element selected from the group consisting of boron and silicon and a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U, the

- process comprising: maintain-ing a bath at a temperature of about 400 to 1000 C., the bath consisting essentially of an alkali metal chloride, and from to 50% of a complex double fluoride of a metal selected from the group consisting of boron and silicon; passing a current through said bath between an anode immersed in said bath and a cathode to deposit on the cathode an alloy of a metal selected from the group consisting of B and Si and of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U; said anode consisting essentially of a mixture of a first carbide selected from the group consisting of boron carbide and silicon carbide and a second carbide of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U.

2. A process for the preparation of an alloy of an element selected from the group consisting of boron and silicon and a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U, the process comprising: maintaining a bath at a temperature of about 400 to 1000 C., the bath consisting essentially of an alkali metal chloride, from 5% to 50% of a complex double fluoride of a metal selected from the group consisting of boron and silicon, and a complex double fluoride of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U, passing a current through said bath between an anode immersed in said bath and a cathode to deposit on the cathode an alloy of a metal selected from the group consisting of B and Si and of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U; said anode consisting essentially of a mixture of a first carbide selected from the group consisting of boron carbide and silicon carbide and a second carbide of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U.

3. A process for the preparation of analloy of an ele ment selected from the group consisting of boron and silicon and a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U, the process comprising: maintaining a bath at a temperature of about 400 to 1000 C., the bath consisting essentially of an alkali metal chloride and 5% to 50% of a complex double fluoride of a metal selected from the group consisting of boron and silicon; passing a current through said bath between an anode immersed in said bath and a cathode to deposit on the cathode an alloy of a metal selected from the group consisting of B and Si and of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U; said anode consisting essentially of a mixture of a first carbide selected from the group consisting of boron carbide and silicon carbide and a second carbide of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U; the mole ratio of the metal content of the first metal carbide to the metal content of the second carbide in the anode corresponding substantially to the mole ratio of the two metals deposited at the cathode.

4. A process for the preparation of an alloy of an element selected from the group consisting of boron and silicon and a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U, the process comprising: maintaining a bath at a temperature of about 400 to 1000 C., the bath consisting essentially of an alkali metal chloride, from 5% to 50% of a first complex double fluoride of a metal selected from the group consisting of boron and silicon, and a second complex double fluoride of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U, passing a currentthrough said bath between an anode immersed in said bath and a cathode to deposit on the cathode an alloy of a metal selected from the group consisting of B and Si and of a metal selected from the group consisting of Ti, Zr, Hf, V,.Nb, Ta, Cr, Mo, W, Th and U, said anode consisting essentially of a mixture of a carbide selected from the group consisting of boron carbide and silicon carbide and a carbide of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U; the mole ratio of the metal content of the first complex double fluoride to the metal content of the second complex double fluoride corresponding to the mole ratio of the two metals in the product deposited at the cathode.

5. A process for the preparation of an alloy consisting of a m'mture of elements selected from one of the following groups: a mixture of boron and silicon; a mixture of an element selected from the group consisting of boron and silicon and a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U; the process comprising maintaining a bath at a temperature of about 400 to 1000 C., the bath consisting essentially of an alkali metal chloride, and from 5% to 50% of a complex double fluoride of a metal selected from the group consisting of boron and silicon; passing a current through said bath between an anode immersed in said bath and a cathode to deposit said alloy on the cathode; said anode consisting essentially of a mixture of carbides selected from any one of the following groups: (a) a mixture of boron carbide and silicon carbide; (b) a mixture of a carbide selected from the group consisting of boron carbide and silicon carbide and a carbide of a metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Th and U.

6. A process for the preparation of a boron-silicon alloy comprising maintaining a bath at a temperature of about 400 to 1000 C., the bath consisting essentially of an alkali metal chloride, and from 5% to 50% of a complex double fluoride of a metal selected from the group consisting of boron and silicon; passing a current through said bath between an anode immersed in said bath and a cathode to deposit said alloy on the cathode; said anode consisting essentially of a mixture of boron carbide and silicon carbide.

No references cited. 

1. A PROCESS FOR THE PREPARATION OF AN ALLOY OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF BORON AND SILICON AND A METAL SELECTED FROM THE GROUP CONSISTING OF TI, ZR, HF, V, NB , TA, CR, MO, W, TH AND U, THE PROCESS COMPRISING: MAINTAINING A BATH AT A TEMPERATURE OF ABOUT 400* TO 1000*C., THE BATH CONSISTING ESSENTIALLY OF AN ALKALI METAL CHLORIDE, AND FROM 5% TO 50% OF A COMPLEX DOUBLE FLUORIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OF BORON AND SILICON, PASSING A CURRENT THROUGH SAID BATH BETWEEN AN ANODE IMMERSED IN SAID BATH AND A CATHODE TO DEPOSIT ON THE CATHODE AN ALLOY OF A METAL SELECTED FROM THE GROUP CONSISTING OF B AND SI AND OF A METAL SELECTED FROM THE GROUP CONSISTING OF TI, ZR, HF, V, NB, TA, CR, MO, W, TH AND U, SAID ANODE CONSISTING ESSENTIALLY OF A MIXTURE OF A FIRST CARBIDE SELECTED FROM THE GROUP CONSISTING OF BORON CARBIDE AND SILICON CARBIDE AND A SECOND CARBIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OF TI, ZR, HF, V, NB, TA, CR, MO, W, TH AND U. 